1. Field of the Invention
The present invention relates to a surface grinding method and apparatus for a thin plate work and particularly, to a surface grinding method and apparatus for a thin plate work such as a semiconductor wafer.
2. Description of the Prior Art
A mirror wafer is generally attained by sequentially performing the following steps of: chamfering for preventing the peripheral region of a wafer which is obtained after passing through a slicing step from chipping; lapping for eliminating a variation in thickness of the wafer; etching for removing a damaged layer and a contaminated portion (where abrasive grains are incorporated); and polishing the chamfered portion of the peripheral region and a major surface of the wafer.
In recent years, a change has occurred in the processing process to attain a mirror wafer, in which change the lapping and etching steps are omitted and instead a grinding step is adopted, whereby a wafer is obtained in a state of being flat to high accuracy and no variation in thickness.
As a processing technique to flatten a wafer, surfacegrinding using a surface grinder has heretofore been known. In the surface grinder, an object to be processed is fixedly held on a rigid chuck table such as a porous ceramic plate and the like, a parallelism between a surface of the object to be processed and a grindstone is adjusted and thereafter, the grinding wheel which rotates is pressed on the wafer to grind off the surface portion of the object to be processed.
In the semiconductor industry, high precision in a silicon wafer, which is an object to be processed, has been demanded: for example, extremely high flatness having a value of 2 .mu.m or less in flatness called TTV (Total Thickness Variation) has been required for a wafer of 200 mm in diameter.
In recent years, an infeed type surface grinder in which a grinding wheel of a cup-shape is set has been used in response to requirement for a wafer surface of high flatness and grinding method has been developed in which a grinding wheel is continuously fed into a silicon wafer to grind while the silicon wafer is rotated about its center at a high speed.
In such a grinding method, a silicon wafer 12 is mounted so that the center of the silicon wafer 12 almost coincides with a rotation center of a rotary table 11 as shown in FIG. 5.
On the other hand, the grinding wheel 6 of a cup-shape is located so that a rotation center of the silicon wafer 12 comes in a working area of the grinding wheel 6.
In this situation, when a relative feed movement is given to the grinding wheel 6 of a cup-shape and the silicon wafer 12 along a direction perpendicular to a working surface being ground while rotating both of the silicon wafer 12 and the cup wheel 6, whole surface of the silicon wafer 12 can be ground without any movement in the grinding plane.
In order to enable high flatness grinding on a wafer, a feed rate of the grinding wheel of a cup-shape is changed in at least three stages, that is high rate feed (depth of cut in grinding), low rate feed (depth of cut) and spark-out (no feed).
However, the following problem still remains in such a conventional technique.
That is, in the grinding method, circumferential speeds in the central portion and the peripheral region of a wafer are different from each other due to wafer rotation about its center; a trace of bending in the shaft of a grinding wheel which rotates the grinding wheel occurs in combination of the speed difference with grinding resistance due to an feed rate of the grinding wheel. The grinding wheel is inclined toward the center side of a wafer surface by the bending and a fault thereby occurs that the wafer and the grinding wheel cannot be maintained in a horizontal plane.
Besides, since, in wafer grinding processing, a high flatness grinding is effected while changing a feed rate (depth of cut) of a grinding wheel of a cup-shape in at least three stages of high rate feed, low rate feed and spark-out (no feed), a further problem occurs that an inclination angle of the grinding wheel also changes according to a change in a grinding wheel feed rate (depth of cut).
The problem will be explained using diagramatic forms in FIGS. 3 and 4. While in the following explanation, three stage feed pattern is shown as an example, there is no specific limitation to the pattern but two stage or more than three stage feed pattern can be adopted.
In FIGS. 3 (a) to 3(b), a grinding wheel 6 drawn in a solid line shows actual grinding postures thereof in operations of various feed rates (depth of cut), while a grinding wheel 6a drawn in a dotted line shows an initial posture when the grinding wheel 6a is set in a surface grinding apparatus. Differences in posture are originated from bending of the shaft of the grinding wheel due to grinding resistance and the like, though the postures essentially coincide with each other if the shaft of the grinding wheel is perfectly rigid.
In the first stage feed of FIG. 3(a), a high rate feed (depth of cut) is adopted taking securement of grinding start and productivity into consideration. At this point, cutting by the grinding wheel 6 occurs toward the c nter portion of a wafer 12 due to grinding resistance which the grinding wheel 6 receives from the wafer and circumferntial speeds in the wafer, the rotary shaft of the grinding wheel is bent corresponding to the cutting and as a result and the grinding wheel 6 is inclined to the central side, so t at ground stock removal in the central side of the wafer s increased as compared with the peripheral region thereof and the wafer 12 comes to have a shape of strong concavity in the ground surface.
Subsequently to the high rate feed, low rate feed (depth of cut) shown in FIG. 3(b) follows in order to enable grinding accuracy on the wafer 12 to be secured with ease. At this point, grinding resistance of the wafer 12 against the grinding wheel 6 is reduced and a bending f the grinding wheel shaft is also decreased in conformity with this, so that inclination of the grinding wheel shaft toward the central side is alleviated, which causes reduction in ground stock removal in the central side of the wafer 12, whereas the inclination of the grinding wheel toward the central side continues and thereby concavity of the wafer 12 is retained, though being shallow.
Further, in FIG. 3(c), no-feed grinding called spark-out is performed. Influences of changes in stress of the apparatus and the material are thereby removed to secure accuracy, but the concave shape of the wafer 12 cannot perfectly be eliminated.
To sum up, as a feed rate (depth of cut) is higher in the initial period of grinding, a trend in which the center of a wafer 12 is more removed happens, so that a shape of the wafer 12 after grinding assumes a bowl-like shape. That is, as a higher feed rate is selected in order to secure higher productivity, a trend for the wafer 12 to assume a bowl-like shape is stronger, so that not only are a time to be spent in the low rate feed and a time to be spent in the spark-out both required to flatten the wafer 12 longer, but the bowllike shape of the wafer 12 cannot be erased with ease, even though spark-out is applied.
Therefore, in a comparative example of the present invention, which is general correction means, as shown in FIGS. 4(a) to 4(c), a posture of the grinding wheel 6 is not positioned horizontal, but in a reverse way corrected being inclined to the peripheral side based on a bowl-like shape of the wafer 12 in spark-out before starting grinding: in a more concrete manner, a grinding wheel posture is initially set so as to correct in the concavity direction by 1 .mu.m based on a shape of after grinding, and then a grinding process to achieve the wafer 12 with high flatness is conducted while changing feed rates (depth of cut) in three stages: high rate feed, low rate feed and spark-out (no feed).
According to such a comparative example, concavities after the high rate feed and low rate feed are decreased by the correction of inclination in posture before the grinding and respectively resulted in 2.5 .mu.m in case of high rate feed and 0.5 .mu.m in case of low rate feed and high flatness can theoretically be secured by grinding 0.5 .mu.m in thickness in spark-out.
However, even in the conventional technique in which correction is effected prior to the grinding in such a manner, the spark-out grinding requires about 10 revolutions and there has been a chance to require a long grinding time in order to correct a bowl-like shape of 0.5 .mu.m, though one or two revolutions are essentially enough. The reason why a grinding time is increased is that a trace of a bowl-shape remains on the working surface even after the low rate feed and the spark-out which essentially plays a role to improve surface finish without any intentional feed has to be utilized to recover a flatness and conduct grinding of a cut depth of 0.5 .mu.m.
Grinding of a wafer 12 in a bowl-like shape and a longer grinding time increases a load imposed on a grinding wheel 6, a working surface of the grinding wheel is worn and loading or grazing, in which no self-truing action for restoration of a cutting ability is exercised, occurs on or in the grinding wheel. A grinding wheel already in such conditions cannot recover an original cutting ability unless a surface portion of the grinding wheel is intentionally removed by abrasion in a process called dressing and this process has had a problem that a life time of the grinding wheel is shortened.